1. Field of the Invention
The present invention relates to astigmatic compensation for an anamorphic optical system.
2. Description of the Related Art
Optical transmission systems employing fiber optic cables are often used to transmit data by means of optical signals. Wavelength division multiplexing (WDM) is sometimes used in such systems to increase the capacity of such fiber optic transmission systems. In a WDM system, plural optical signal channels are carried over a single silica based optical fiber with each channel being assigned a particular wavelength. Dense WDM (DWDM) is also increasingly being used.
Erbium-doped optical fiber amplifiers (EDFAs) are often used to amplify light before transmitting the amplified optical signal to the input of an optical fiber of an optical transmission system. EDFAs contain a single-mode optical fiber doped with erbium. The erbium-doped fiber is "pumped" with light at a selected wavelength to provide amplification or gain at wavelengths within the low loss window of the optical fiber. Optical systems and EDFAs typically utilize transmission fibers with a circular core. The input light pumped into to the erbium-doped fiber of the optical amplifier is typically provided by a pump laser module comprising a semiconductor laser diode (pump laser) plus an associated lens system.
A pump laser module typically comprises a pump laser such as a semiconductor laser diode fabricated in a given substrate such as InP or GaAs; an optical lens system for focusing and optically processing the beam; and a fiber for receiving the beam and outputting the beam. The pump laser typically receives an input signal in the form of an electrical current, and outputs an optical beam on the fiber. This fiber is typically fusion-spliced into a single-mode fiber of a wavelength division multiplexing (WDM) device. The fiber of the WDM device may be fusion-spliced to a single-mode erbium doped optical fiber of an optical amplifier. The WDM device combines the pump light and signal light and outputs this to the single-mode erbium doped optical fiber of the optical amplifier. The optical amplifier thus receives coherent light from both pump and signal laser source, and amplifies the relatively low signal light to higher power before transmitting the amplified optical signal to the input of an optical fiber of an optical transmission system. Semiconductor pump laser diodes have therefore become very important components for modem optical communications systems.
There are a variety of semiconductor lasers that emit an elliptical beam. For example, many GaAs-based lasers operating at wavelengths such as 650, 780, 810 and 850 nm all emit an elliptical beam. In particular, a conventional 980 nm GaAs laser, often used as a pump laser source for optical amplification systems, emits such an elliptical beam.
DWDM is emerging as the technology of choice for the backbone of the Internet due to its cost effectiveness in carrying huge data. DWDM-based systems utilize either 980 nm or 1480 nm pump lasers. The shorter wavelength pumping provides a better noise figure, while the longer wavelength provides a higher signal output. The demand for optical pump power in a single mode fiber increases every year. For a DWDM system, for example, the demand for 980 nm pump power has been more than doubled within the past five years from 60 mW to 150 mW.
The 1480 nm pump lasers fabricated from an InP substrate have a near circular cross-sectional output laser beam. However, as noted above, the 980 nm pump lasers fabricated on a GaAs substrate typically have an elliptical emission aperture and thus the laser beam has an elliptical shape or pattern. The 980 nm pump laser has gained popularity due to its high electrical-to-optical conversion efficiency and the low noise figure for the fiber amplifiers. However, since the associated optical systems utilize transmission fibers with a circular core, much of the launched power may be lost. Conventional optical systems will capture only approximately 40% of the launched power from an elliptical beam.
Referring now to FIG. 1, there is shown a schematic diagram illustrating the optical layout of a conventional optical lens system 100 of a pump laser module. Optical design and related matters are discussed in further detail in W. M. Sherry, C. Gaebe, T. J. Miller and R. C.
Schweizer, "High Performance Optoelectronic Packaging for 2.5 and 10 Gb/s Laser Modules," Proc. of Electronic Components and Technology Conference, pp. 620-627, 1996.
The pump laser of FIG. 1 is formed from 980 nm laser diode 101, and comprises lenses 102, 103, 104. Ball lens 102 is positioned in front of laser diode 101 to collect the divergent beam produced by the diode. The beam is collimated after it passes through ball lens 102. A field lens 103 is disposed to focus the collimated beam, and the beam thereafter passes through a focal point F and again begins to diverge. The divergent beam is then intercepted by a third lens 104 that corrects for spherical aberration and re-focuses the beam. When a conventional 980 nm pump laser beam propagetes through a system such as conventional optical lens system 100, the horizontal far-field angle may vary from 6.degree. to 12.degree. (FWHM), and the vertical far-field may vary from 20.degree. to 35.degree. (FWHM). An exemplary pair of angles may be 9.degree. for the horizontal and 30.degree. for the vertical. This give rise to an elliptical beam pattern. As the beam propagates through this system, the ellipticity of the beam is not corrected.
Referring now to FIG. 2, there is shown an illustration 200 of an elliptical beam pattern 201 produced by the conventional optical system 100 for a 980 nm laser diode 101. As shown in FIG. 2, the typical mode-field radii for 980 nm pump lasers are 1.9 .mu.m and 0.7 .mu.m for the horizontal (X-axis) and vertical (Y-axis) directions, respectively. When the laser beam passes through optical system 100, the exit beam pattern 201 is elliptical, as shown in FIG. 2. The mode field radii predicted by optical design software simulations are 8.5 .mu.m in the X axis and 4.6 .mu.m in the Y axis. Since the mode-field radius for a typical single mode fiber at 980 nm is 3.2 .mu.m, a significant amount of the optical power will not be coupled into fiber 106. The simulated (calculated) coupling efficiency for system 100, with a typical 980 nm laser 101, is only 59%.
Thus, 980 nm pump lasers often exhibit poor coupling efficiency due to the elliptical beam pattern produced. The main reason for this ellipticity is the relatively weak index waveguide provided by the ridge waveguide laser structure in GaAs substrates. Although a strong index guided waveguide can be easily fabricated in InP, it cannot be easily fabricated with GaAs, the material used to fabricate 980 nm lasers. Thus, with a comparatively weak index waveguide, the pump laser requires wider width, and the aspect ratio between horizontal and vertical apertures becomes large, implying an elliptical-shaped beam. The ratio for 980 nm (GaAs) pump lasers is typically from 2.5 to 5.
Pump lasers with a large aspect ratio will result in a poor coupling efficiency (CE) when the light is coupled to a single-mode fiber (e.g., of an EDFA) by using conventional optical design, because the single-mode fiber requires a circular mode field pattern for high CE. The larger the aspect ratio is, the more degradation there will be in the coupling efficiency. For typical 980 nm lasers, the degradation can be severe with more than 3 dB penalty in the fiber-coupled power. The primary reason for this degradation is the mismatch in the mode field diameters along the major and minor axes of the beam.
Several schemes have been proposed to improve this coupling efficiency. One approach, based on a virtual point source lens, corrects the mode mismatch but requires very tight angular tolerance. Another approach, based on a wedged fiber lens, also corrects the mode mismatch but requires very tight offset tolerance. These approaches are described, respectively, in Virtual Point Source Optics for Laser Diodes (Blue Sky Research, 1994) &lt;http:.backslash..backslash.www.blueskyresearch&gt;; and R. A. Modavis & T. W. Webb, "Anamorphic Microlens for Laser Diode to Single-Mode Fiber Coupling," IEEE Photonics Tech. Let., Vol. 7, No. 7, pp. 798-800, 1995.
The wedged fiber micro-lens solution to this coupling efficiency problem, for example, which employs an anamorphic fiber micro-lens, can achieve a CE of about 70%. However, the microlens requires very stringent alignment tolerances. Additionally, the ellipticity of the launched beam may vary, depending upon the manufacturer (for example, the ratio of far-field angles between the vertical and horizontal directions can vary from 2.5 to 4.5, depending upon the manufacturer). Thus, even the best anamorphic microlens design may not be suitable for all situations.
There is a need, therefore, for an improved 980 nm pump laser and means for improving the coupling efficiency of elliptical beams into a single-mode optical fiber.